1. Field of the Invention
The present invention relate to a sheet feeding device, an image forming apparatus, and an image forming system.
2. Description of the Related Art
As a method which allows a sheet feeding device used in an image forming apparatus to separate a sheet from piled sheets and convey the sheet, an air separation method which sucks air to draw an uppermost sheet of a piled sheet bundle onto a suction belt is proposed.
Japanese Laid-open Patent Application No. 2010-120721 discloses a sheet feeding device including an air blowing unit configured to suspend multiple sheets of a sheet bundle placed on a sheet stacking member above by directing air toward side faces of a top portion of the sheet bundle in three directions (a front face and both side faces in a width direction of the sheet bundle). The sheet feeding device sucks air to draw an uppermost sheet of the multiple sheets suspended by the air blowing unit onto a suction belt, thereby separating and conveying the uppermost sheet. In Japanese Laid-open Patent Publication No. 2010-120721, an optical reflection sensor which, when the air blowing unit starts blowing air, detects presence or absence of a sheet at a sheet feeding position where the uppermost sheet of the sheet bundle is to be positioned is provided. When a ratio of a period, during which the optical reflection sensor detects presence of a sheet, in a predetermined period after the multiple top sheets of the sheet bundle are suspended by the air blowing unit is equal to or smaller than a preset value, the sheet stacking member is lifted a predetermined amount.
Japanese Laid-open Patent Publication No. 2010-120721 describes the reason why the control operation which lifts the sheet stacking member based on the ratio of the period, during which the optical reflection sensor detects presence of a sheet, in the predetermined period is performed is as follows. The multiple sheets suspended by the air blowing unit behave unstably in such a manner that they occupy various transient positions between the sheet bundle and the suction belt. Therefore, performing a control operation of lifting the sheet stacking member when the optical reflection sensor detects absence of a sheet and stopping lifting the sheet stacking member when the optical reflection sensor detects presence of a sheet causes disadvantages. The disadvantages are, more specifically, that the sheet bundle excessively approaches the suction belt and frequency of multifeeds is increased and that the sheet bundle is excessively separated from the suction belt and sheet feeding capability is decreased.
Japanese Laid-open Patent Publication No. 2010-120721 puts a focus on the following correlation that the closer an uppermost sheet of a sheet bundle which is not suspended (hereinafter, “non-suspended sheet bundle”) to the suction belt, the larger the ratio of the period, during which the optical reflection sensor detects presence of a sheet, in the predetermined period when the air blowing unit is blowing air, and employs the control operation which lifts the sheet stacking member based on the ratio of the period, during which the optical reflection sensor detects presence of a sheet, in the predetermined period. This control operation makes it possible to position the non-suspended sheet bundle in a predetermined area, thereby reducing sheet feeding failure.
However, relation between position of the uppermost sheet of the non-suspended sheet bundle and the ratio of the period, during which the optical reflection sensor detects presence of a sheet, varies depending on sheet conditions including an environment where the device is used (hereinafter, “usage environment”) and a sheet type. For this reason, there is disadvantageous in that, because it is necessary to make an assessment of relation between position of the uppermost sheet of the non-suspended sheet bundle and ratio of the period, during which the optical reflection sensor detects presence of a sheet, in various sheet conditions including usage environments and sheet types in advance, manufacturing cost will increase. Furthermore, because it is necessary to set the preset value for use in determining whether or not to lift the sheet stacking member the predetermined amount for each of the sheet conditions including the usage environments and the sheet types, a necessary capacity of memory where the preset values are to be stored disadvantageously increases. Furthermore, a detecting unit(s) for detecting a usage environment, a sheet type, and the like are required, the number of parts will disadvantageously be increased, which leads to an increase in cost of the device.
Under the circumstances, there is a need for a sheet feeding device and an image forming apparatus capable of reducing sheet feeding failure while reducing an increase in manufacturing cost and cost of the device.